Many applications require precision clocks or oscillators, which are hereinafter collectively referred to as oscillators. For example, many radar systems require precision oscillators that exhibit phase noise that is as much as 180 dB/Hz below the carrier frequency. Consequently, before an oscillator can be integrated into such a system, the oscillator must be tested to determine if oscillator meets the phase noise requirements of the system.
Presently, there is an analog technique that is capable of measuring phase differences that are as much as 180 dB/Hz below the carrier or reference signal. The analog technique involves applying the signal of a DUT and a local oscillator signal to a mixer. The mixer operates to produce an intermediate frequency signal. The intermediate frequency signal is low-pass filtered to produce a signal that is proportional to the sine of the phase difference between the signal of the DUT and the local oscillator signal. If the signal of the DUT and the local oscillator are maintained at or near quadrature, then the sine of the phase difference is approximately equal to the phase difference and the intermediate frequency signal is approximately proportional to the phase fluctuations between the signal of the DUT and the local oscillator signal. The signal of the DUT and the oscillator are maintained at or near quadrature by a control loop, such as a phase-locked loop or controllable delay line.
A digital approach to measuring phase differences involves applying the signal of a DUT to an analog-to-digital converter to produce a digital signal. The analog signal is sampled in accordance with a clock signal generated by a sampling clock. The digital signal is digitally multiplied by digital versions of an in-phase local oscillator signal and of a quadrature local oscillator signal. The product of each of the multiplications is subjected to digital low-pass filtering to produce an in-phase signal and a quadrature signal. Application of the in-phase and quadrature signals to a phase detector that calculates the arctangent of the quotient of the quadrature signal divided by the in-phase signal yields a signal that is indicative of the phase difference between the signal of the DUT and the local oscillator signal. An advantage of this approach relative to the analog approach is that the phase detector does not require two signals that are at or near quadrature to produce a signal that is indicative of the phase difference between the signal of the DUT and the local oscillator signal. The DUT is compared against the internal sampling clock over multiple periods of the arctangent function with no degradation in performance, provided there is sufficient numerical precision to store the total elapsed phase and still resolve noise.